Sealed bearing roller cone drill bits generally incorporate a compensating mechanism to limit the pressure differential between the lubricant sealed within the drill bit and the well bore fluid. The most common device is a flexible diaphragm separating the two fluids. The diaphragm responds to the conditions in the well to maintain a balanced pressure across the primary dynamic seals in the drill bit.
As previously mentioned, these devices also typically compensate for volumetric changes of the lubricant which occur in the form of leakage or through thermal expansion.
Previously constructed compensators have been generally successful in performing the desired functions. Bit manufacturers currently supply acceptable compensators. However, difficulty is encountered during assembly due to the relatively large number of separate components that must be carefully inserted in the correct sequence into each cavity in each arm of each drill bit. Most of the compensators utilize some form of rolling diaphragm that isolates the drilling mud from the bit lubricant. The rolling action of the diaphragm compensates for volumetric changes in the lubricant.
U.S. Pat. No. 4,276,946 issued Jun. 7, 1986 illustrates one previously constructed compensator that also utilizes the resiliency of the diaphragm to assist in determining when the proper quantity of lubricant has been injected into the system.